Carbon fibers are greatly useful for an aerospace material, a sports material, a leisure material, and an industrial material for a pressure container or the like due to the excellent mechanical strength thereof and are demanded in many fields. Further, it is expected that carbon fibers are used for a broad field in the future.
In general, the carbon fibers are manufactured by carbonizing a carbon fiber precursor fiber bundle, obtained by binding a precursor fiber of a carbon fiber such as a polyacrylonitrile fiber, at a high temperature. Specifically, the carbon fiber precursor fiber bundle is subjected to a heating treatment (a flame-resistant treatment) in a flame-resistant furnace filled with an oxidization atmosphere so as to obtain a flame-resistant fiber bundle, and the obtained flame-resistant fiber bundle is subjected to a heating treatment (a pre-carbonizing treatment and a carbonizing treatment) in a carbonizing furnace filled with an inert atmosphere such as nitrogen so as to obtain a carbon fiber.
In general, the carbon fiber is not directly used, and is formed as a composite material by the combination with a matrix resin so as to be used for various applications.
However, when the wettability, the compatibility, and the adhesiveness of the carbon fiber and the matrix resin are not sufficient, it is difficult to obtain satisfactory properties when the carbon fiber is used as a composite material.
For this reason, a surface treatment is generally performed on the carbon fiber subjected to the heating treatment in the inert atmosphere so as to modify the surface of the carbon fiber and a sizing treatment is further performed thereon, whereby the wettability, the compatibility, and the adhesiveness with respect to the matrix resin are improved.
As a method of performing the surface treatment on the carbon fiber, there is known an electrolytic oxidation treatment method, a liquid-phase oxidization treatment such as a chemical oxidization treatment, or a gas-phase oxidization treatment. When the oxidization treatment is performed on the surface of the carbon fiber, an oxygen-containing functional group is formed on the surface of the fiber, and hence the wettability, the compatibility, and the adhesiveness with respect to the matrix resin are improved. Furthermore, the carbon fiber subjected to the oxidization treatment is generally subjected to a sizing treatment by a sizing agent after the carbon fiber is dried inside a dryer or the like.
Even in such an oxidization treatment, particularly the electrolytic oxidation treatment is a surface treatment method that is practical and effective compared to the chemical oxidization treatment or the gas-phase oxidization treatment from the viewpoint of the easiness of the treatment, the easiness of the treatment condition control, and the easiness of the introduction of the oxygen-containing functional group with respect to the carbon fiber surface.
However, in the electrolytic oxidation treatment method, there is a need to clean the electrolytic solution adhering to the carbon fiber after the electrolytic oxidation treatment by pure water or the like. As a result, the manufacturing apparatus may increase in size or the manufacturing cost may increase.
Further, when drying the cleaned carbon fiber, there is a need to dry the carbon fiber at a temperature equal to the boiling point of solvent used in the electrolytic solution or higher than the boiling point thereof in consideration of the electrolytic solution that remains after the cleaning treatment. Since sulfuric acid or nitric acid is generally used in the electrolytic solution, there is a need to use a high-temperature dryer capable of setting a temperature equal to the boiling point thereof, and the manufacturing cost easily increases.
Further, when the electrolytic solution used for the electrolytic oxidation treatment is directly drained, environmental burden increases. For this reason, there is a need to perform a waste liquid treatment such as a neutralization treatment so as to prevent the environmental burden. As a result, the productivity is degraded, and the manufacturing cost easily increases. Since the cleaning waste liquid having been used to clean the carbon fiber after the electrolytic oxidation treatment contains the electrolytic solution, the same problem occurs even when the cleaning waste liquid is drained.
Therefore, as a carbon fiber surface treatment method that does not require a cleaning process, is convenient in operation, and reduces manufacturing cost, for example, Patent Document 1 discloses a method of treating a surface of a carbon fiber by immersing or transporting the carbon fiber into or inside an ozone solution obtained by dissolving ozone.